{  Statement: http://acmp.ru/?main=task&id_task=554
   Verdict: Accepted
}
uses
    SysUtils, Math;

var
    i,n:longint;
    ans:int64;
    
begin
reset(input,'input.txt');
rewrite(output,'output.txt');
read(n);
ans := 2;
for i := 2 to n do
    inc(ans, i);
write(ans);
end.